Radio location system



Aug. 27, 1963 R. C. wlLsoN ETAL 3,102,266 I RADIO LOCATION SYSTEM original Filed Dec. 1o, 195e 7 Sheets-Sheet 1 Aug. 27, 1963 R. c. wxLsoN ETAL RADIO LOCATION SYSTEM '7 Sheets-Sheet 2 Original Filed Deo.

llllllllllllrlllllnllll|vlllll|l INVENTORS ROBERT C.WILSON ond BY ROBERT J. PUCHATY MWKMWIQMMMWW- ATTORNEYS Aug. 27, 1963 Original Filed Deo. l0, 1956 7 Sheets-Sheet 3 Osc OSCILLATOR CONTROL CIRCUIT-8O I I f/0/ @3e/5; 54( I BUFFER AMPLIFIER PI-IAsE l AMPLIFIER I, L STAGE INVERTER AMPLIFIER "h 3. j 3) l oscILLAToR \52 ,f -7- /02 /0 /04 I 600m f l 79 I @.5 f j I 24" FROM 7 0 aesqs 0 SIGNAL MONITOR 90 /74 I soo-mou I mPLIIII-IIII?E PusH- I DP PULL I BRE 92 78 INVERTER AMPLIFIER I Ogg-Ig-" J To) E ,7/2 /72 MonuLAroR F L /7i cIRcuITs-Is V- ToNE DETECTOR-74 I /84 ol. BPF. D c. graob 'AMPL'F'ER soo/v -RECT'F'ER AMPLIFIER @grazi I aoj /5/j A92J /55J "'jI. i

rONDITION ,96 GREEN INDICATOR I /67 I I 77 /94 /d I T 6.5 v -T Q89 '[95 "I A.c. RED I l @5m-md.

ro ALARM o ob/S /92 HL/ Dc. /86 fgfgf AMPLIFIER I Si RECTIFIER AMPLIFIER 'I I l Y l L TPQECIO R 5 J INVENTIIRS A ROBERT C. WILSON und VWIIIMI ATTORNEYS Aug. 27, 1963 R. c. WILSON ETAL RADIO LOCATION SYSTEM '7 Sheets-Sheet 4 Original Filed Deo.

INVENTORS ROBERT C. WILSON und BY ROBERT J. PUCHATY ATTOREEYS Aug. 27, 1963 R. c. WILSON ETAL RADIO LOCATION SYSTEM 7 Sheets-Sheet 5 Original Filed Dec.

m20... .5002 zmmw INVENTORS ROBERT C.W|LSON 0nd BY ROBERT J. PUCHATY our NNN

AT TO RNEYS Aug. 27, 1963 R. C. WILSON ETAL RADIO LOCATION SYSTEM Original Filed Dec. l0, 1956 7 Sheets-Sheet 6 END STATION-3H 334 I .354 END STATION-SI2 550 I 350) 55I"=l| 353) Y 555) 55Al l TRANsNI'T'R TRANsIIITR Q:'/R l I fg/.R TRANsMrR WQBOOOKC- WQBQOOKC UNIT l I UNH I'IQBBOOKG I y L l L f .9 -36T`ONE TRAIISIFER 332 35 555 Z TRAIISIFER Tod/E 5I DETECTOR UNITAND f I 35 i UNIT .AND L I DETECTOR R.F. OETEO; 360 Rf DETEC. I llaa? 340 591 Il QPLTR -REcEIvER RECEIVER #gg I 240 N ITIZOOOKC 358 1 I gynngookc y156C.) v -l ANTENNA ITRANSIIITRII TRANSMTRCHANGEWR CENTER STATION BIO l I I79s24OI c Ivsemoxc. UNIT 320 3/7) gj I I TT 522 v 5,26 I I TONE II'NFI 325 z/ Y I l5 DETEcTOR -RE DETEC' `FILTER FILTER FILTER RECEIVER l l 52.?) 7 240- 36o/v 60o/5J mzooo Kc f l l I zal ""XER I Il I l 73M ANTENNA '/372 I cHANGEOv'R UNIT 5872 574' IIIISIII I AMPLIFIER MOOULATN DETECTOR 577 370 II y I 5/ E TRANsMTR 7 VTRANsNITR l 240A, ITTEOOOKO' I Inaoooxc 1= L BEF. E 575 ssofv REFERENCE 361) 560 575 I STATION SYSTEM FAILURE REFERENCE 976 -313 IIIDCIIIRNTIT 'NDCATOR ""Irggslgc I l II\IVEI\TI R5 I A ROBERT C. WILSON and FIG. 7

www, @AW QAIIIMEAIIIIM BY ROBERT J PUCHAIVY ATTORNEYS Aug. 27, 1963 R. c. WILSON ETAL RADIO LOCATION SYSTEM '7 Sheets-Sheet 7 Original Filed Dec.

ATTORNEYS United States Patent 3,102,266 RADIO LOCATION SYSTEM Robert C. Wilson, Iowa City, Iowa, and Robert J.

luchaty, Tulsa, kla., assignors to Seismograph Servlee Corporation, Tulsa, Okla., a corporation of Delaware Original applications Dec. 10, 1956, Ser. No. 627,398, 110W Patent No. 3,019,435,1lated Jan. .30, 1962, and Ser. No. 800,845, tiled Mar. 2,0, 1959. Divided and this application Apr. 21, 1960, Ser. No. 23,676

7 Claims. (Cl. 343-105) llhe present invention relates to radio position iinding systems and more particularly to improvements in radio position finding systems employing phase comparision in` (now Patent No. 3,019,435) and 800,845, respectively.

filed December l0, 1956i, and March 20, 1959, and assigned to the same assignee las the present invention.

'ln systems of the particular type referred to, the continuous waves radi-ated from each pair of transmitters as received at the receiving point have a phase relationship which changes as a function of changing position of the the receiving point between the two transmitting points. More specically, the waves radiated by each pair of transmitting units tof the system are characterized by spaced isophase lines which are hyperbolic in contour about the transmitting points as foci. On a line connecting the pair of transmitters, these isophase llines are spaced apart a distance equal to one-half the mean wave length of the radiated waves and have div'erging` spacings at points on either side of this line. With this system arrangement, the posi-tion of a receiving point relative to a pair of hyperbolic isophase lines may be determined by measuring the phase relationship between continuous 'waves radiated from the pair of transmitters.

Since the point `of location of the receiving point along the zone separating the two isophase lines is not indicated by such a phase measurement, it is desirable to employ at least three space transmittersfdiiferem pairs of which function to provide a grid-like pattern of intersecting hyperbolic lines, in order to obtain absolute determination of the position of the receiving point. Systems of the character described are exceedingly accurate insofar as the position indications produced at the receiving point are concerned. To obtain che desired indication accuracy, however, it is necessary to maintain phase synchronization between the continuous waves radiated by the spaced transmitters, or, alternatively, so to arrange the system that phase shifts between the radiated Waves are compensated during the phase comparing operation.

Phase synchronization of the waves radiated from the plurality of transmitters presents an exceedingly ditticult problem which has been the subject of considerable development work. All solutions which have been found for this problem involve the use of relatively elaborate and somewhat delicate instrumentation not well adapted for the continuity of service required in position determining systems. To obviate this problem, systems of the continuous wave hyperbolic type have been proposed (see Honore Patent No. 2,148,267, issued February 21, 1939) in which the phase shift problem is obviated by heterodyning the carrier Waves of each pair of transmitters at a fixed link transmitting point, and modulating the difference. frequency component of the heterodyned waves as -a reference signal upon the carrier output [of the link transmitter for radiation to the receiving point, where the difference frequency component is detected and phase compared with a difference frequency signal derived by directly heterodyning the transmitted continuous waves at the receiving point. In this manner, phase shifts between the continuous waves radiated'from the two transmitters are completely compensated so that the measured phase angle is truly representative of the location of the receiving point between a pair of isophase lines.

One system which utilizes the principles of the Honore patent but which reduces the number of transmitting stations and frequency 'channels required is disclosed in United States Patent No. 2,513,316 to i ames E. Hawkins. In the system disclosed and claimed in the latter patent, a common link or reference transmiter is employed for modulating the reference signals developed upon a single space radiated carrier wave. Another arrangement which has ybeen used to reduce both the amount of equipment and the number of frequency channels is disclosed and claimed in United States Patent No. 2,513,317 to lames E, Hawkins and Robert S. Finn. -In the system disclosed in the latter patent two Iof the position signal transmitters are alternately rendered effective as link or reference signal transmitters under the control lof signals alternately radiated from a third position signal transmitter. While these systems provide satisfactory operation, it has been necessary, until the present invention, to maintain operating personnel at each of the transmitting stations in order to turn the equipment on or olf whenever desired and to make certain that the equipment is functioning properly during use. Obviously, the use tof such a large number of operators materially increases the cost of system operation.

The present invention concerns a system in which the operation `of the entire system may be monitored at a single point in such manner that, in the event of operational failure, the source `of the trouble may be identified and maintenance personnel may immediately be dispatched to correct fthe diiculty. Thus, the present invention makes it possible to dispense with the use of operating personnel at all of the stations except the control station. In addition, .the present invention provides a convenient system for periodically checking the system operation at all the mobile receivers in order to determine whether phase shifts have occurred either in the transmitting equipment or the receiving equipment.

Accordingly, the principal object of the present invention is to reduce the number of persons required to maintain operation of a transmitting `system fof the hyperbolic, continuous wave type.

A further object of the present invention is to provide a transmitting system of the hyperbolic, continuous wave type which requires the continuous services of an operating crew at only one of the stations of the system.

It is also an object of the present invention to provide a transmitting system of the hyperbolic, continuous wave type in which the oper-ation of the system is continuously monitored ata single station so that operational failures can be detected immediately.

Another object is to provide a transmitting system according to the preceding object in which the monitoring provides `an indication of the source of the operational failure so that the trouble can be corrected without delay.

It is likewise an object of the present invention to provide ya transmitting system of the character indicated above wherein the system may be periodically calibrated to deterrnine whether phase shifts have occurred either in the transmitting or receiving portions of the system.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the speci- .a fication taken in conjunction with the accompanying drawings 4in which:

FIG. 1 is a ydiagrammatic representation of a three-foci transmitting system of the hyperbolic continuous wave type embodying the present invention;

FIG. 2` is a diagrammatic representation of the tone detector, the antenna changeover unit and the transfer unit and modulation detector employed at one of the end transmitting stations of the system illustrated in FIG. l;

FIG. 3 is a diagrammatic representation of the oscillator control circuit and of the signal monitor unit employed at the center transmitting station of the system illustrated in FIG. l;

FIG. .4 diagrammatically illustrates the tone detector units Vand thecon'dition indicator employed'at the center transmit-ting station of the system illustrated in FlG. 1;

t PIG. 5 is a diagrammatic representation of the transfer unit employed at the center transmitting station of the system illustrated in FIG. y1;

/ FIG. 6 4is a diagram-matic representation of the equipment employed at the network oice of the system illustrated in` FIG. l;

FIG. 7 is a ldiagrammatic representation similar to FIG. 1 but illustrating another embodiment of the present inp vention; yand FIG. l8 diagrammatically illustrates the failure indicator circuit employed at the control station of the system shown in FIG. 7.

Referring now to the drawings, and, more particularly, to FIG. 1 thereof, the invention is there illustrated as `embodiedin a three-foci hyperbolic, continuous Wave system lfor providing position information -at any number of mobile receiving units (not shown) which may be carried by. vessels or Vehicles operating within the radius of transmission of a plurality of spaced apart transmitting units or stations .10, 11 and 12. The transmitting system illustrated in FIG. l is somewhat similar to that disclosed land claimed in lthe above identified U.S. Patent N o. 2,513,- 317 to Hawkins and Finn but, yof course, additionally includes the remote control and monitoring equipment of the present invention. Thus, the transmitting units 111 and 12 `are `spaced approximately equal distances `from the location of the transmiting station 10 and, as a result, the transmitting station `10` may be referred to as a center transmitting station, while the stations 11 and 12 may be called end transmitting stations. To facilitate the ensuing description the end transmitting station y111 will at times be referred to as the Red end station and the receiving equipnient designed to receive the waves radiated from this station will be termed `a Red receiver while the end transmitting station 1,12 'will be referred to as a Green station and the receiving equipment responding -to the waves from station 12 will be termed a Green receiver. The stations '11 and 12 are so positioned that an imaginary base line interconnecting the points of location of units 10 and 11 is angularly related to an imaginary base line interconnecting the points of location of the uni-ts 110 and `12. As: is

from an understanding of the above identified Hawkins Vand Finn patent, the center transmitting unit 101 is equipped alternately to radiate a pair of position indicating signals so separated in frequency that they may be selectively received and distinguished. The end transmitting stations .11 and '112, on 4the other hand,'1are equipped continuouslygto radiate position indicating signals which are alternately modulated with reference signa-ls developed in a manner described more fully hereinafter.

f Specifically, the center transmitting station '16* comprises a firstk transmitter or transmitting unit indicated generally at 114 and a second transmitter or transmitting unit indicated generally at 15. Only one of these transmitters is employed at a particular time, the remaining transmitter v being. a standby or alternate unit which is automatically rendered effective by a transfer unit 16 in the event of transmitter -failure at the center station. Since the transmeans of a commutator 21 which is driven from a synchronous motor and gear train Z2, as Vfully described in the Hawkins and Finn patent referred to above. rFhus, as illustrated in lFIG. 1, the oscillators 17 and 1S are alter- Vnately keyed by'alt'ernately supplying anode current Yt'o their electron discharge tubes from the positive terminal 93 of a power supply 94 through a commutating ring 21a which is shaft-connected as indicated at 95 to be driven at constant' speed by the synchronous motor and .gear train unit 22. More specifically, the positive terminal 93 of the anode current source is connected to the'conductive segment 12t1b of the commutating ring 21a, which segment spans slightly less than hal-f the circumference of the ring. rlfhe remainder of the ring is composed of an insulating segment 21C. At diametrically opposed points on the periphery of the ring are provided brushes 21d and 21e fwhich are respectively connected to positive conductors leading to the oscillators 17 `and 18. Since the conductive segment 2lb of the commutating ring Zlaywhich segment the peripherysurface, it `will be understood that'a short offsi-gnal period is provided between successive periods durmg |which the oscillators 17 and 18` are alternately operated, thus preventing simultaneous radiation of the two waves developed by these oscillators. The periodicity with which the vtwo oscillators are alternately operated is, of course, dependent upon the speed of rotation of the cornmutator ring. Preferably, this ring is driven at a speed' of one revolution per second, such that the oscillators 17 and 1S are each rendered operative at one-half second intervals.

The oscillator 17 in the transmitter 14 and the corresponding oscillator in the transmitter 15 may also be turned on by means of a manually operated switch 96 which, whenclosed, bypasses the eommutator and connects the terminal 93 directly to Vthe `electron discharge tubes of the oscillators. Similarly, a manually operated switch 97 is provided to bypass the commutator and turn on the oscillator 1S of the transmitter 14 and the cor-responding oscillator yof the transmitter 15. There is also provided yan on-off switch 98 for controlling the delivery of A.C, current to the power supply 94 and to theremaining circuits at the center transmitting station 10. A

p switch 99 may be employed, if desired, to control the delivery of energizing voltage for the synchronous motor and gear train units of the transmitters 14 and-15. Thus, in order to render the center station effective to radiate lcon-tinuouslya 1772.000 kilocycle signal during calibration or testing of the equipment, the switches 96 and 98` are both closed. If, ron the other hand, it is desired to radiate continuously Ia 1798.000 lnliocycle signal from the center station for test purposes or for calibration, the switches 97 and 98 are both closed. To provide the normal operation :of the center transmitting station 10 in which 1772.000l1and 1798.000 kilocycle signals are alternately radiated in the manner described abo've, the switches 96 and 97 are left in their I.open position and the switches 98 and 99 are closed. The output signal from the final amplifier of each of the transmitters 14 and 115 is passed to the transfer unit and hiatus reducer 16 which selects the signal to be radiated and operates an antenna changeover unit 23 to pass the selected 'signal `to an emitting antenna 24. The antenna changeover unit 23 samples the signal output of the unit 16 and supplies a signal to the unit 16 'which informs the latter to switch to the standby transmitter in the event that signal output of the unit This alternate operation of the oscillators 17 and 18 may be effected by 16 disappears. In view of the foregoing description, it will be recognized that, regardless of which of the transmitters 14 or y15 is in operation, the center transmitting station is effective alternately to radiate position indieating signals having frequencies of `1772.000 and 1798.000 kilocycles.

The equipment provided at the end station 11 includes a first transmitter 30 and a second transmitter 31, both of which are adapted to develop signals having a frequency of 1798.600 kilocycles. 11n the event of failure of one of the transmitters, the remaining transmitter, termed a standby transmitter, is 'adapted to be placed in operation automatically by means of a transfer unit and modulation detector 321 which functions to pass the sign-al developed by the operative transmitter through an antenna changeover unit 33 to a radiating or emitting antenna 34. Transmitters 30 and 31 are identical and each includes an oscillator or signal generator indicated by the reference numeral 35 in the transmitter 30. This oscillator develops signals having a frequency of 1798.600 kilocycles for passage through an amplitude modulator 36 and through a final amplifier 37 to the transfer unit and modulation detector 321. The antenna changeover unit 33 also samples the output signal of the end transmitting station 11 and supplies this sample ato the unit 32 so that in the absence of transmission signal, the unit 32 4will automatically render the standby transmitter effective.

For the purpose of developing reference signals which are alternately supplied to the modulator circuits of the transmitters 30 and 31, the end .station 11 also comprises a pair of receivers 33 land 39 which are identical in Eonstruction, which are both tuned to a center frequency of 1772.120 kilocycles and which are both maintained'in continuous operation, sothat in the event of failure of one of the receivers the otherreceiver will continue to effect proper operation of the equipment at the end station 11. The receivers 3S and 39 are excited by .signals picked up by a common loop receiving antenna 40 which is so oriented that signals radiated from the antenna 34 are nulled or eliminated, thereby minimizing receiver blocking problems which might otherwise result `from the radiation of high power signal from a point relatively close to the receiving antenna. The signals arriving from the center station 10 and from the end station 12, of course, initiate a response in the antenna itl and lare passed to the signal input terminals of both of the receivers 38 and 39. The output of the receiver 3S is passed through a band pass filter 41 tuned to la frequency of 240 cycles, while the output of the receiver 39 is passed through a similar band pass filter d2. As will be described more fully hereinafter, the receivers 38 and 39 are adapted to heterodyne tor beat the signa-ls arriving from the center station 10 and from the end station 12 in order to develop the 240 cycle difference frequency therebetween,

which 'difference frequency is, of course, passed through the lters 41 and 42 to .the modulator circuits of the transmitters 30 and 311. The signals passed by the filters 41 and 42 are also applied to a tone detector circuit 43 where they function in a manner to be described more fully hereinafter to control `the `oper-ation of the transfer unit and modulation detector 32 in order to permit the end transmitting station 11 to be turned on lor olf from a remote point.

The equipment provided at the end station 12 is similar to that at the end station 11, except, of course, for the frequency of oper-ation of certain components of the rtransmitting and receiving circuits. Thus, the equipment at the end station 112 includes a first transmitter 50 and a second transmitter 51 `which are identical in construction and which are adapted to be lrendered effective one at a time under the control of =a transfer unit and modulation detector S2. The signals developed by the effective transmitter are passed through an antenna changeover unit 53 to a radiating antenna 54. The antenna changeover unit 53 samples the output of the transmitting station V12 and supplies a signal to the unit 512 so that, in the absence of sigral output from the station 12, the standby transmitter is rendered effective automatically. The transmitter 50, which, as indicated above, is identical to the transmitter 51, comprises an oscillator :or signal generator `55 for developing signals having a frequency of 1772.240 tkilocycles which are passed .through lan amplitude modulator circuit 56 and through `a final amplifier 57 to the transfer unit and modulation detector 52.

For the purpose of heterodyning the signals received from the center station 10 and lfrom the end station 11 to develop the reference signals referred to above, the end station 12 is provided with a pair 0f wave signal receivers 53 and `5'9` of iden-tical construction, both`center tuned to a frequency of 1798.300 kilocycles and both maintained in continuous operation. The input circuits of these receivers are each excited by the signals developed by a loop receiving antenna 60 which, of course, is so oriented that the signals radiated from rthe antenna 54 are nulled in order to eliminate or minimize the receiver blocking problems referred to above. The antenna 60 is, of course, excited by the signals arriving from the center station 10 and from the end station 11 and these signals are passed to the input of receivers 58 and 59. The receivers 5S and 59 are each adapted to heterodyne the signals received Vfrom the center station 10 and from the end station 11 and |to develop the 600 cycle difference frequency therebetween. The latter difference frequency is passed through 600 cycle band pass filters 61 and 62 to the modulation circuits of the transmittersfStl and 51, where it is amplitude modulated upon the signal developed by the oscillator circuits of these transmitters. The signals passed by the filters 61 and 62 are also applied to a tone detector unit 63 for controlling the operation of the transfer unit and modulation detector 52 in order to permit the end station 12` to be turned on or off from a remote point as described below. A

In view of the foregoing description, it will be recognized that either the transmitter 30 or the transmitter 31 at the end station 11 is rendered effective to cause continuous radiation of a single carrier Wave signal having a frequency of y1798.600 kilocycles, either the transmitting unit 14 or the transmitting unit 15 at the center station is operative to radiate alternately signals having frequencies of 1772.000 and 1798.000 kilocycles, and either the transmitter `Si? or the transmitter 51 at the end station 12 is effective to radiate continuously signals having a frequency of 1772.240 kilocycles. During the interval when the center station 10 is radiating its 772.000 kilocycle signal, the receivers 33 and 39 Zat the end station `11 both heterodyne the signal from the center station with the signal continuously radiated from the end station 12 to develop a 240 cycle beat frequency which, as previously described, is passed through the filters 41 and d2 to the modulator circuit of the effective transmitter at the end station 11. Thus, the signal radiated from the end station 11 is alternately modulated with a 240 cycle reiference signal. The loop receiving antenna 40 cooperates with the receivers 38 and 39 to prevent the appearance of the signal radiated from the end station 11 at the output of the receivers 38 and 39. The modulated carrier wave radiated by the end station 11 is, of course, accepted by the receivers 58 and 59 at the end station 212, but the 240 cycle reference signal has no effect on the operation of the end station 12 in view of the fact that it is rejected by the filters 61 and l62. 'llhis reference signal may, if desired, be reproduced at the end station 12 and used during installation of the system in Calibrating to adjust the frequencies of fthe transmitters 50 and 51 'for monitoring or for other similar purposes.

During the interval when the center transmitting station is effective to radiate signals having a frequency of 1798.000 kilocycles, the receivers 58 and 59 at the end station 12 lieterodyne the signal received from the center station with the signal continuously radiated from the end station 11 in order to develop a 600 cycle beat frequency which is passed through the filters `61 and 62 to the modulating circuit or the effective transmitter at the end station 12. rllhus, the signal radiated by the end station 12 is alternately modulated with reference signals having a frequency of 600 cycles. The latter reference signal appears at the output of the receivers 38 and 39 at the end station 11 and may be used for monitoring or calibrationas described above, but it is prevented from affecting the operation of the end station 11 due to the fact that it is rejected by the filters 41 and 4,2,

The signals radiated from all three of the transmitting units 10, 1.1 and 1 2 are received at a mobile receiving unit of the type described in the above-identified Hawkins and Finn patent where the received signals are employed to develop a pair of position indications representative of the location of fthe mobile receiving unit relative to the three transmitting stations. Thus, las will be apparent from an understanding of the Hawkins and Finn patent, the 24()- cyele reference signal modulated upon lthe carrier Wave radiated from the end station 11, the carrier wave continuously radiated from the end station 12 and the 1772.00() kilocycle wave radiated from the center station l are employed to develop a first position indication representative of the location of the mobile receiving unit relative to hyperbolic isophase lines having foci at the center station lll and at the end station 12,. Similarly,the 600 cycle reference signal modulated upon the carrier wave radiated from the end station 12, the carrier wave vradiated from the end station 11 and the 1798.000 kilocycle wave radiated from the center station 1li are used to provide a position indication representative of the location of the mobile receiving unit relative to hyperbolic isophase lines having foci at the center station 10 and at the end station 11.

. In addition to the two transmitting units 14 and 15 previously described, the center station 10 also includes means for monitoring the signals radiated from the transmitters to provide a continuous check on Ithe operation of the system, to enable the operatorto evaluate the quality of emission and to facilitate frequency adjustment so that the desired audio beat frequencies can be maintained. Moreover, the center station 10' includes yapparatus for cooperating with the equipment at the end transmitting stations to effect remote control of the latter stations so that the entire transmitin-g sysem may be turned on from .the center station without requiring the presence of operators at the `two end stations. In addition, a monitoring station indicated by the reference numeral '70 may be employed either at the transmitting station 10 or at a location remote from all three of the transmitting stations, such as at the network ofhce, to monitor the system in order .to ascertain Whether the equipment is functioning properly and to locate the source of trouble in the event that failure occurs.

Speciically, the center transmitting station 1li comprises, in addition .to the equipment previously described, a pair of receivers 71 and 72 respectively tuned to frequencies o2 1772.24() kilocycles and 1798.600 kilocycles, both of which are excited by signals received at a loop pickup antenna 73. The lantenna 73` is, of course, oriented so that the signals emitted from the end stations 11 and 12 are accepted and applied across the signal input terminals of bothof the receivers 711 and 72 while the signals emitted from the antenna 24 are nulled and eliminated. The receiver 71 is `suiiiciently selective to reject the l798.600 kilocycle signal arriving from the station 11 while accepting the 1772.240 kilocycle signal arriving from the end station 12. The 60G cycle modulation component of the signal received from the end station 12 is reproduced by the receiver 71 and is passed through a band pass filter 92 to a pair of tone detector circuits '74 and 75'. The receiver 712, on the other hand, is sufficiently selective fto reject the signal received by the antenna 73 from the end station 12, but the signal received from the end station 11 is -accepted `and the 240 cycle modulation component thereof is reproduced and passed through a 24() cycle band pass filter 76 to the tone detectors 74- and 75. The tone detectors 74 land 75 are employed to control the Operation of a condition indicator circuit 77, which indicates the absence of one or both of the signals from the filters 76 and 92. The signals passed by the filters 76r and 92 are also applied through one section 78 of an oscillator selector switch 79 to la `signal monitor 90 described more fully below.

To provide for initiation of the operation of theend stations 11 and 12 from the center station 10, there is provided at the latter station 'an oscillator control circuit indicated generally at 80 for developing audio frequency signals to lbe modulated uponythe wave radiated from the center station. This oscillator control circuit includes a pair of highly stable audio frequency generators S1 and 32 for developing signals having frequencies of 240 cycles and 60() cycles, respectively. The outputs of these two signal generators are respectively applied to spaced apart contacts 83a and 83h of section 83 of the switch 79. The switch 7 9 rnayl he employed to apply the output of either the `generator 81 or the generator 82 through an amplifier 84 and through a modulation `on-oif switch d5 to the modulator circuits of the transmitters 14 and 15. In

addition, the switch 35 applies the output of the amplier 84 to the signal monitor 96' where it is compared with the audio signal supplied through the switch section 78.

Essentially, the remote control equipment comprises means including the oscillator control circuit 80 for modulating the 1772.000 kilocycle signal radiated from the center stati-on 1@ with a 240l cycle tone switching signal, means .at one of the end stations,.for example, at the station 11 for receiving the switching signal and, after a predetermined time delay, turning on the end station 114, after which the end station 1.2 will be turned on automatically in response to the 1798.000 kilocycle signals alternately radiated from the center station and to the 1798.660* kilocycle signal emitted from the end .station 11. The end station 12 may also be turned onby the modulation of a 600 cycle tone modulation signal upon the 1798.000 kilocycle signal radiated from the center station lll. The circuits at the end station' are so designed that the tone switching signals must :be applied throughout the predetermined delay on period, thus preventing the transmitting stations from being turnedfon by spurious noises or the like. However, the time constants of thesecircuits are such that the' normal switching rate taking place lat the center station and produced by the commutator 21 does not affect the operation. By proper use of the facilities provided, the center station operator may turn on either of the end stations without turning on the other for purposes of testing, calibration or the like, `or he mayturn on both of the end stations in order to place the entire transmitting system in operation. The end stations may also he turned off automatically merely by rendering the center station inoperative, thereby to terminate the development of tone signal-s at the end stations. Even after the tone signal development has ceased, the end stations remain on for a .predetermined delay olf period somewhat in excess of the delay on period for purposes developed more fully below.

Turning now Vto the oscillator control circuit 80' provided at the center station 10 for developing the tone switching signals referred to above :and referring particularly to FIG. 3 yof the drawings, it will be observed that this circuit includesa pair of audio oscillators 81 and 8-2. rThese oscillators are of the feedback type having a vibrating reed in the `feedback circuit :for stabilizing the frequencies of the generated signals. As indicated above, the oscillator 81 is designed and adjusted to develop an audio frequency signal of 240 cycles, while the oscillator 82` develops `an `audio frequency signal of 600L cycles. To increase the stability of the oscillators and to immunize them from load changes, their outputs are applied through a conventional buffer amplifier 101 to section 813 of the oscillator selector switch 79. Specically, the 240 cycle signal developed =by the oscillator 81 :appears between contact 53a of switch section 83 and ground, while the 600 cycle Isignal developed Iby oscillator 82 appears between contact 83!) and ground. Thus, the movable arm or pole of switch section 83 may be :selectively controlled to apply either the 2.40` cycle or the 600 cycle frequency yto the ampliiier 84. The latter amplifier comprises an input arnpliier stage 102 having its signal input terminals connected to the movable arm of the switch section 83 and a phase inverter circuit 103` for developing push-pull signals for application to lan output push-pull ampliiier d of conventional construction. The signal output terminals of the push-pull amplifier 1M are connected through the modulation `on-oti :switch 85 to the modulator circuits of both of the transmitters 14 and 15. The switch 35, when closed, is also eiiective to deliver the output signals of the push-pull ampliiier 1M to the signal monitor 9u described more fully below. In view of the foregoing description, it will be recognized that by the selective manipulation of the oscillator selector switch 79 the modulator circuits of the transmitters 11i- :and 15 maybe supplied either with a 240 cycle tone switching signal or with a 600 cycle tone switching signal.

' Let it be assumed for the present that it is desired to turn on the end station `11 while, at the same time, maintaining the end station 12 in its inoperative condition. Such a condition might be desirable for purposes of monitoring the end station 11, for calibration, to facilitate frequency adjustment and the like. To effect the desired result, the oscillator selector switch 79 is so manipulated that the signal appearing between contact 33a of switch section 85 and ground is applied to the input of the amplifier S4, the modulator on-oi switch 85 is closed, the oscillator control switch 96 is closed and, of course, the main power on-oii switch 98 is closed while switches 97 and 99 remain open. in this manner the center station is rendered eective continuously to radiate a 1772.000 kilocycle carrier wave which is modulated by the 240 cycle signal developed by the oscillator control circuit Sil. This modulated carrier wave signal is, of course, rejected by both of the receivers 58 and 59 at the end transmitting station 12 and, hence, has no effect on the operation of the latter station. The 24() cycle audio switching signal is, however, reproduced by both of the receivers 38 and 39 at the end transmitting station 11 and is passed through filters @1 and 42 to the tone detector circuit 43. As previously mentioned, both of the receivers '38 and 39 are maintained in operation continuously so that each will be conditioned to receive the signal from the center station. The two signals passed by the iilters 41 and 42 are applied as illustrated in FIG. 2 to separate sets of signal input terminals of a linear mixer circuit y1616 in the tone detector 43. The linear mixer circuit functions to mix the'two input signals Without developing sum and diierence frequencies and passes both of the 240 cycle signals to an amplifier circuit 167. The output of the `latter amplier is passed through a 240 cycle audio band pass iilter `16S to a half-wave rectiiier circuit 109. The latter rectiiier circuit clips off the positive half-cycles of the 240 cycle input signal and passes the negative half-cycles through a resistancecapacitance iilter network to the control grid of a DC. amplifier 110. The time constant of this resistancecapacitance iilter network, as described more fully below, is sufficient to maintain the negative signal on the grid of amplifier 110 during a period corresponding to the normal switching rate eiected by the commutators in the transmitters 14 and 1S at the center station. The negativesignal applied to the control grid of the D.C. ampliiier 1'10 is suliicient to cut ofr the plate current of this amplifier and, since this plate current tlows through the operating coil 111 of a t'one detector relay `112, it will be apparent that whenevery 240 cycle signals are passed by the iilter 10S, the relay 1112 is opened or deenergized. The audio iilter 108 in the tone detector prevents spurious signals or interference from opening the tone detector relay and eiiectively prevents opening of this relay except during periods when a 240` cycle tone signal is being developed by receivers 38 and 39. The linear mixer 106 in the input of the tone detector circuit allows the relay 112 to be opened when a signal is developed either by the receiver 38 or the receiver 319. Thus, the tone detector 43 will respond to a 24() cycle signal developed by either one of these receivers, even though the other receiver may be ineffective due to operational failure or the like. In any event, prior to the development of the 240 cycle switching signal by the receivers 33 and 3.9, the D.C. amplifier 110 is in a conducting state and its plate current iiow energizes the relay 112 to hold pole 11201 in engagement with stationary contact 112k. As soon as a 24() cycle signal is developed by either of the receivers 3S or 39 as, for example, by reproducing `the 240 cycle tone switching signal modulated upon the 1772.000 kilocycle wave being continuously radiated from the center station 1t? under the described conditions, the relay 112 opens and pole 112er dropsv into engagement with a normally closed contact 112e, thereby applying voltage to a time delay relay 113 which may be referred to as the delay on relay. Specifically, when the relay -112 is opened, one terminal 114 of an alternating current source is connected through the relay 112, through a signal connector 115', through the normally closed Contact 113m of the time delay relay -113 to a time delay motor 116 or the like in the relay 113. The other side of the time delay motor 116 is permanently connected through a conductor 117 to the second terminal 118 of the A.C. source.

Thus, as soon as relay 112 opens, the time delay motor is energized and the timing cycle of the delay on period begins to run. After a predetermined delay which, in accordance with a particular installation ot the present invention, is set at 60 seconds, the operating coil i119 of the relay`113 becomes energized to move the pole 113b of the relay into engagement with normally open contact 113e, thereby applying the line voltage across the operating coil 121i of a latching relay 12,1. The latter relay is of the type having its cont-acts mechanically latched both in the open and closed positions and, hence, the relay contacts remain closed. The time delay :relay 113 is of the instantaneous reset type, so that if the signal supplied to the DC. amplifier is lterminated during the delay on period for a period of time in excess of the time constant of the RC network in the input circuit of this ampliiier, the relay 112 is closed and the relay 113 is reset so that the timing cycle must be re-initiated. The instantaneous reset feature of the relay 113 thus prevents short or intermittent spurious signals such as static or the like from operating the latching relay 121. When the coil 12111 of the latching relay is energized, contacts 1-21a and 121b are closed to complete a circuit through a ratchet relay 122 `to one of the transmitters 30 or 31 at the end station 111. The condition or position of the ratchet relay will, of course, determine which one of the transmitters is turned on initially, Thus, for example, when the ratchet relay is positioned as illustrated in FIG. 2, the power supply for the transmitter 31 is delivered through a bus conductor 123, through the now closed contact 1:21a, through a connector 124, through closed contact 1272er of the ratchet relay, through a signal connector `125, through a signal light 129 for indicating that transmitter 31 is on, through the operating coil .126 of an antenna changeover relay 127 and through a connector '1218 to the plate and screen circuits of the electron discharge tubes of the transmitter 31. Plate voltage is not delivered to the electron discharge tubes of the transmitter 39, in view of the fact that contact 122b of 11 the ratchet relay 1.22 is open.` The iiow of current through the operating coil 126 of the antenna changeover relay closes contact 127e and connects the output of the iinal amplifier of thetransmitter 31 through a coaxial cable 130 to the antenna 34.

With the circuits of the transmitter 31 energized, the 240 cycle signal reproduced by either or both of the receivers 3S and 39 is passed to the modulator circuit of the transmitter 31, where it is amplitude modulated upon the carrier wave radiated from the antenna 34. A portion of the signal supplied to the antenna 34 is applied through a suitable coupling circuit or sampling means to a modulation detector circuit indicated generally at 138. This coupling circuit may comprise a small capacitor connectedY to the inner conductor of the coaxial cable 130 for supplying signals through a connector 132 to the signal input circuit of a detector 131 at the input of circuit 138. The latter detector is of conventional construction and functions to demodulate the input signal and to reproduce the 240 cycle modulation component which is passed through a transformer coupled amplitier 133 vto a half-wave rectifier 134. The rectifier 134 is similar to the rectifier 109 described above and functions to eliminate the positive half-cycles and to pass the negative half-cycles of the 240cycle signal to the control grid of a D.C. amplifier 135. The latter amplifier, like the amplier 110 described above, includes in its input circuit a resistance-capacitance network having a time constant sucient to maintain the negative voltage during the normal switching rate of the transmitting system. The negative voltage supplied by the rectifier 134 has a magnitude suicient to cut oil? the amplifier 135 whenever signals are supplied to the cable .130. rI'hus, when the 240 cycle signal is modulated upon the wave radiated from end station 11, the amplifier 135 is cut oli and the plate current flow to the operating vcoil 136 of a modulation detector relay 137 is terminated. Thus, before the latching relay 121 has been energized to turn on one of theV transmitters at the end station 11 the amplifier 135 is in a conducting state, the relay 137 is actuated and its contact 13`7a is closed. As soon as a 240 cycle signal is n tiated. The time-delay effected by the relay l140 is considerably less than that provided by the delay on relay 113 and, in accordance with a particular embodiment of the present invention, is adjusted to seconds. Thus, at the end of the 5 second delay period the operating coil 142 of the relay 140 is energized to close ythe normally open contact 14% in order to apply line voltage across the operating coil 143 of the ratchet relay 122. Actuation of the coil 1'43 draws an operating pawl 144 into engagementwith the teeth of a ratchet wheel 145 to advance the relay 122 one step. The ratchet 145 is ixedly mounted Ona shaft indicated at 146 which also carries a pair of switch operating cams 147 and 14S having cam teeth which are so spaced that the contacts `122e and 122b will eachy be alternately opened and closed as the ratchet relay is advanced. Specically, whenthe operating coil 143 is actuated to advance the ratchet relay from the position illustrated in FIG. 2, the contact 122:1 will be opened, due tothe fact that movable arm 149 will drop into one of the recessed regions on the periphery of the cam 147. At the same time, contact 122b is engaged by arm 150, due to the fact that this arm is moved radially outward by engagement with a toothed region on the cam 148. When the contact-'122e is opened, of course, the plate voltage supply to the electron dischtrge tubes of the transmitter 31 is interrupted and this transmitter is,

of course, rendered ineffective. Closingfof the contact f122b conditions the plate supply 'circuit for the transmitter 30 to be completed as soon as the latching relay 121 is operated at the completion of the delay on period effected by the relay 1,13. Thus, at the start of each turn-on period at the end transmitting station. 11, the ratchet relay 122 is operated to transfer from one of the transmitters 30 or 31 to the other transmitter. In this manner, extended use of a signal transmitter over a long period of time is prevented and the operating life of'each of the transmitters 30 and 31 is lengthened.

Some time after the ratchet relay 122 has been operated in the manner described above, the timing cycle of the delay on relay 113 will be completed to operate the latching relay 121 in the' manner described above, whereupon a circuit will be completed from bus conductor 151 through the now closed contact 12112, through a connector 152, through the now closed contact 122b of the ratchet relay 122,`through a signal lightr154 and through a signal connector 153 to the plate and screen grid circuits of the electron discharge tubes vof the transmitter 30, thereby to place the latter transmitter in operation. The on condition of the transmitter 36, of course, is indicated by illumination of the signal light 154.

During calibration or for test purposes, the transmitters time delay circuits and the various relays described above;

Specitically, when the switch `155is moved to a downward position, illustrated in FIG. 2, a direct connection is provided between conductor V151 and conductor 153, thereby delivering plate voltage to the tubes of the transmitter 30 to place the lattertransrnitter in operation. During this time, of course, the transmitter` 31 is not operated due to the fact that the lower section of the switch 155 is confronted by an open circuit condition. Under these conditions, the antenna changeover relay 127 is not energized and the output of transmitter 30 is connected to the antenna circuit. When, however, the switch 155 is moved to lthe upper `position illustrated in FIG..2, the connection between conductor 151- and conductor 153 is bro-ken and the conductors 123 and 125 are connected directly together to deliver plate voltage to the tubes of the transmitter 311. Under the latter conditions, the antenna changeover relay 112.7 is operated to transfer the antenna circuit connection from the output of the, transmitter 3l) to the output of the transmitter 31.

To summarize briefly the turn-on procedure described above, it will be observed that the presence of the 240' cycle tone switching'signal at the output of either of the receivers 38 or 39 operates the tone detector circuit to close the relay 112. The latter operation starts the time delay relays 113 and A140. At the end of a delay period of a few seconds, the relay140 is etiective to operate the ratchet relay 122 and condition the operating circuits for the transmitters 3@ or 31. Some time later, the delay on relay 113 is effective to energize the latching relay 121 andvcomplete the connection to one of the transmitters 30 or 31 through the ratchet relay 122. Thus, each time the end station 1v1 is turned on, the ratchet relay 122. and its associated circuitry are effective to' transfer from one of the transmitters 301 or 31 to the other transmitter.

When thelatching relay 121 has been energized and oneV of the transmitters 30 or 31 has been placed vin operation, the antenna changetwer relay 127 is automatically rendered effective to connect lthe rantenna circuit to the energized transmitter.

. Whenthe station 11 has been turned on, the modulation detector circuit 138 opens the relay 137 to break the circuit to the relay 140. The relay y137' remains open as long as a modulated'signal is present on the cable 1351 and, hence, the circuit at the end station will remain at the described position. If, however, fthe'signal on the cable 136 disappears as, for example, by failure of the eftective transmitter at theend station 11, the negative voltage suppliedV Y 13 to the control grid of the D.C. amplifier '135 disappears to render this tube conducting and to 4close the modulation detector relay 137. At this time it will be remembered that a 24() cycle signal is still present at the output of the receivers .33 and 39 and, hence, tone detector relay 112 remains open. As soon as relay 137 is closed, the timing motor 14,1 of the time de-lay 1401 is energized. If, during the delay period provided by the relay 146, the equipment failure is corrected, that is, if the trouble occurred as a result of momentary power failure or the like at the station 11, the relay 137 is immediately opened and the time :delay relay 146` reverts to its original condition without having any eiect on the operation of the ratchet relay 122 or the other circuits illustrated in FIG. 2. If, however, the failure does not correct itself within the delay period provided by the relay 141i, the relay operating coil 142 is energized to close contact ldilb, thereby to advance the ratchet relay 122. The latching relay 121 is, of course, closed at this time and, accordingly, the advancement of the ratchet relay immediately places the standby transmittcr in operation, thereby automatically energizing the standby. transmitter in the event that the eifective transmitting equipment fails.

When it is desired to turn off the end transmitting station 11, the operator at the center station may remove the 240 cycle modulation signal as, for example, by opening the modulation on-of switch 85. When this modulation is removed, the `output `from the receivers 38 and 39 is interrupted and the D.C. amplifier 11@ lis no longer excited by negative signals from the rectifier 1w. Therefore, the ampliiicr 110 begins to conduct and its plate current oiw energizes the relay l112 to close contact 11211 in order to complete a connection through conductor 157 and through normally closed contact 15`8a of a delay oit time delay relay 15S to the timing motor 159 of the latter relay. The delay oit period effected by the relay 153 is considerably longer than the delay on period provided by the relay 113 and, in accordance with a particular installation of the present invention, this delay period was selected at 90' seconds. During this delay oit period the end transmitting station 11 remains in operation so that if the absence of signal output at receivers 38 and 39' is due to a momentary failure of the equipment either at the center transmitting station 10 or at the end transmitting station 11, the operation of the end transmitting station can be resumed as soon as the momentary failure is corrected.' Thus, in the event that 240 cycle signals are again produced by either of the receivers 38 or 39 rduring the 90 second delay oit period, the relay 112 is again opened and the delay off relay 158 reverts to its normal position, illustrated in FIG. 2. The delay oft circuit thus prevents lthe transmitting equipment fat the end station 11 from shutting down in case of momentary power failure at the end station 1i1, thereby to provide a period of time sufficient for automatic starting type generators to pick up the load and correct the power failure. In addition, the delay oit circuit holds the end station 11 on the air for a period of time which is suiiicient to iallow the entire system to be placed into operation during a complete turn-on period as described below. At the end of the delay off period, the operating coil 160` of the relay 158 is energized to close contact 1581 and, hence, to energize the turn ofi operating coil 1162 of the -latching relay 121 via conductor 4161. Energization of the coil 162 releases the mechanical llatch holding the movable arms of the relay 121 in the down position and draws these arms upwardly, thereby interrupting both of the circuits supplying power to the transmitters 3d and 31. Thus, both of these transmitters are rendered inoperative and the end station 11 goes off the air.

The equipment provided at the end transmitting station 12 to eiect the automatic transfer between the transmitters Si) and 51 and to provide .the remote control operation is similar to the `equipment just described at the 1d transmitting station 11. Thus, the tone detector 63 is identical to the tone detector 43 just described except, of course, that the audio tilter in the detector 63 is tuned to pass signals having a frequency of 601() cycles, while the audio lter 108, as indicated above, passes 24()l cycle signals. The frequencies of operation of the remaining circuits lat the end station 12 are changed in similar manner. Therefore, in the event that it is desired to turn on the end station 12 for test purposes or for calibration while, at the same time, maintaining the end station 11 in inoperative condition, the operator at the center station may ieave switches 96 and W open, close the main power on-oi switch 93, close the manually operated switch 97', close the modulation on-ol switch 8S and place the oscillator selector switch '79 in position to deliver 600 cycle signals from the oscillator control circuit e@ through the amplifier vSit to the modulator circuits of the transmitters 14 yand 15. In this manner the center station 11i is rendered effective continuously to radiate a 1798.00() kilocycle carrier wave amplitude modulated with a 601i) cycle switching signal. This signal is received by the antenna 60' and is passed to the receivers 58 tand 59 where the 600 cycle modulation component is detected and passed through filters 61 and `62 to the tone detector 63. The tone detector 63 responds to the input signal by opening its tone ydetector relay in order to turn on the transmitting equipment at the end station 12 in exactly the same manner as the turn-on procedure which takes place at the end transmitting station 11 and which has been described previously. The end transmitting station 12 is, of course, turned oit by removing the 6G() cycle tone switching signal modulated upon the carrier wave radiated from the center station as, :for example, by opening the modulation on-oif switch 85. In the absence of signal output yat both of the receivers 5S and 59, the tone detector relay is closed and the transmitting station 12 is turned on in exactly the same Amanner as the turn-oli procedure employed at the end station 11. The transfer unit 4and modulation detector 52 is supplied with a sample outputfrom the antenna changeover unit 53` in order to etiect automatic transfer to the standby transmitter in the event or operational failure of the main transmitter in the manner previously described. Also, the unit 52 -is effective to transfer from one of the transmitters Sii or 51 to the other transmitter each time the end station equipment is turned on in order to instrument the transmitter equipment-sharing feature previously described.

Assuming that the transmitting station 11 has been 501 'turned on in the same manner described above by continuously radiating from the center station 1111 a 1772.000 kilocycle signal modulated with a 246 cycle tone detector signal, the 240 cycle signal modulated on the wave radiated from the end station 111 is detected and is passed through the band pass iilter 76 and through :section 78 of the modulation on-o switch 79 to the signal monitor 90 in the manner illustrated in FIG, 3. Specically, with both sections of the switch 79` in their upper position, illustrated in FIG. 3, the output of the band pass iilter '76 is developed across a gain control potentiometer 170. This potentiometer may be varied to control the amplitude of the input signals supplied to an ampliiier and phase inverter unit 17,1 which develops push-pull signals for exciting a conventional push-pull amplifier `172. The output of the push-pull ampliiier i172 is applied to one set of deflection plates of an oscilloscope i173, the other pair of deilection plates of which are excited by the 'signal output of the push-pull ampliiier Y104?, supplied through switch 85. Thus, a lissajous pattern is produced on the screen of the oscilloscope in response to the two 240 cycle signals respectively applied to/ its horizontal and vertical plates. As previously indicated, the 2.40 cycle signal developed by the oscillator 81 is highly stable and, hence, serves as a frequency standard. Thus, the lissajous pattern may be observed by the operator at the center station 10 to make certain that the transmitting equipment at the end station 11 is functioning properly and also to facilitate calibration and frequency adjustment of the transmitting equipment at the center station. The output of the push-pull amplifier v172 may also be applied to a loudspeaker or the like in order to produce a continuous 240 cycle tone which indicates that the end station E11 has been turned on and that a modulated carrier wave is being received from this station. l

Similarly, whenv the end station 12 has been turned on by continuously radiating from the center station 10 a 1798.000 kilocycle carrier wave amplitude modulated with a y600 cycle signal in the manner previously described, the modulated wave radiated from the antenna 54 is received by the receiver 71 at the center station 101. Thelatter receiver detects the 600 cycle modulation component and passes it through the band pass filter 92 and through the push-pull amplifier 172 to the oscilloscope 173 where itis compared with the highly stable 600 cycle signal developed by the oscillator 82. Obviously, the 600 cycle signal appearing at the output of the push-pull amplifier 172 is also applied to the loudspeaker 174 to indicate that the end station y12 is on the air and that the receiver 71 is receiving its amplitude modulated carrier Wave.

`In the event that the signal monitor 90 indicates that the end transmitting station being monitored has failed to come on the air, it is only necessary to remove the tone switching signal by opening the switch 185 for a period of several seconds, for example, 10 seconds, and then to reapply the modulaton by again closing the switch.

` When the tone switch signal is removed for a l0 second period, the tone detector relay in the tone detector at the end station closes. ,This breaks the circuit to the time delay relays 113 and 140 and, (accordingly, when the tone switching signal is reapplied at the end of the l0 second interval, the relay 112 opens to apply a signal through the closed contacts of switch 137 to the time delay relay 140.5 Obviously, the relay 1,37 is closed, in view of the fact that the end station 11 has failed to come on the air. After the delay period produced by the relay i140, the ratchet relay122 is advanced to condition the transmitter transfer circuits so that at the completion of the delay cycle eifected by the relay 1.1-3 the operation will be automatically transferred to the standby transmitter. The operator at the center station can then dispach a maintenance crew to repair the inoperative transmitter so that both of the transmitters at the end stations will again be ready for operation.

in the event that it is desired to turn on both of the end stations 1'1 and 12 to place the entire system in operation, the center station il()1 is rendered `effective alternately to radiate its two carrier waves of `1772.000 kilocycles and 1798.000 kilocycles and one of these waves is modulated with a tone switching signal supplied by the oscillator control circuit 80. To eiect this operation, the operator closes the switches 98 and 99 and leaves the switches 96 and 97 open. If it is desired to initiate operation by turning on the end transmitting station 11, theV oscillator selector switch 79 is positioned to supply a 240 cycle signal through the amplifier 84 land through the closed switch 85 to the modulator circuits of both of the tnansmitters 14 and 15. The commutator 21, as previously described, is effective to render the oscillators of these transmitters alternately operative to develop the two carrier wave signals. The 240 cycle signal supplied from the amplifier control circuit 80 is thus modulated upon both of the radiated signals. The 240 cycle signal 16 modulated upon the 1798.000 kilocycle wave is reproduced by the receivers 58 and 59 at the end station `12 but is rejected by both of the iilters 61 and 62 and, hence, has no elfect on the operation at this end station. The 2A0 cycle signal modulated upon the'1772.000 kilocycle wave radiated from the center station is reproduced by the receivers 38 and 39 at the end station 11 and is passed through the lilters 411 and 42 to the tone detector 43 in order to turn on the end station 1:1 in the manner described above. While the 240 cycle signal appears at the output of the receivers 38 and 39 only during the alternate intervals of operation of the center transmitting station 10, as described above, the resistancecapacitance network at the input circuit of the D.C. amplier 1110 in the tone detector 43 holds over for this switching period and, hence, maintains the tone de-k tector relay 112 in its open position as long asy the tone switching signal is being received. As soon as the transmitting station =1'1 is placed on the air to radiate its 1798.600 kilocycle signal, the receivers '58 and 59 at the end station 12 heterodyne the signal continuously radiated from the end station 11 with the 1798.000 kilocycle signal alternately radiated from the center station `l0 to develop a 600 cycle signal which is passed through the filters 61 and 62 to operate the tone detector relay 63 in the manner indicated above, thereby placing the end transmitting station 12 on the air. When the end station 12 is placed in operation the receivers 38 and 39 at the end station l1'1 function to develop :the 240 cycle beat frequency between the signal continuously radiated, from the end station 12 and the 1772.000' kilocycle signal alternately radiated from the center transmitting station r 10. Thus, the -tone switching signal developed by the oscillator IS0 can be now removed by opening switch 85 and the outputs of receivers 38 and 39 vat station 111 and of receivers 58 and `59 at station 112 will maintain the end stations in operation. As previously indicated, in the event of tnansmitter failure of a momentary nature at one of the end stations, .the delay off relay I158 in the tone detector circuits at the end stations holds the transmitter ofthe other end station on the air so that if the failure corrects itself within the second delay period continul ous operation is maintained.

The system may also be turned on by supplying a 600 cycle tone switching signal from the oscillator control circuit 80 for modulation upon the carrier waves.v alternately radiated from the center station. tion 12 is turned on iirst and end station 11 comes on Vthereafter in response to action of receivers 38 and 39 Aand passes this signal through the lter 76 to operate the tone detector 75. Since the signals produced by receivers 71 and 72 are used only for monitoring purposes and have no effect on the position indications provided at the mobile receiving unit, it is not necessary to employ standby receivers at the center station.

As illustrated in FIG. 4, theA tone detectors 74 and 75 are similar to the tone Adetector i43-previously described. However, in view of the fact that the input circuit of each of the tone detectors 74 and 75 is excited by a single signal, a linear mixer need not be employed as the input stage. Thus, the tone detector 74 includes an ampliiier circuit which is excited by the 600 cycle signals from the band pass lilter `92. The output signals of amplifier 180 are passed through another 600 cycle band pass filter In this case, end sta- 181 to a hlalf-wave rectifier circuit 182 which is similar to the rectiiier 109 described above. Thus, the rectiiier 182 passes the negative half-cycles of the 600 cycle signal to the control grid of a D C. `ampliier 183 which is identical to the amplifier 110. The RC network in the input circuit of the D.C. -amplitier 183 has a time constant which permits this circuit to hold -and maintain the negative signal during the normal switching period of the system and thus as long as signals lare supplied to the tone `detector 74, the tone `detector relay 184 remains open. The tone detector 75 is identical to the tone detector 74 except that Ia 240 cycle band pass filter 185 is employed in place of the lilter 181 and, as a consequence, as long as the tone detector 75 is excited by a 240 cycle input signal from band pass lter 76 the tone detector relay 186 is maintained open.

The tone detector relays 184 and 186 are `employed to control the operation of the condition indicator 77 which provides appropriate indications in the event of failure of one or more of the lend transmitting stations. As long as the transmitting system is functioning properly, the relays 184 and 186 remain in their open position, illustrated in FIG. 4, and current from an A.C. source is supplied from terminal 192 through a manually operated reset switch 191 through signal connector 190 and through the contacts of relays 184 and y186 to one side of system-on signal light 196 and to one side of operating coil 194 .of `'an alarm relay 195. The other sides of coil 194 'and of light 196 are connected together and to terminal 188 of the source. Thus, the relay 195 is normally energized and light 196 is illuminated lfor normal system operation.

However, in the event that the 600 cycle signal input to the tone detector 74 disappears the relay 184 will be closed to close Contact 184er and complete a circuit for energizing a sign-al light 187. The described circuit extends from terminal 188 through a signal connector 189, through the signal light 187, through the contact 18%, through connector 190 and through the reset switch 191 to the terminal 192. Thus, the absence `of 600 cycle signal input to the tone detector 74 is indicated immediately by illumination 'of the Green signal light 187.

ln the event that the 240 cycle signal input to the tone detector 75 disappears, the relay 186 is closed to complete la circuit to a signal light 193. ln the event that both of the tone detectors 74 and 75 lose their input signal, the circuit to the operating coil 194 of the alarm relay 195' is opened and the circuit to the system-on light 196 is also opene-d. The relay 195 has two sets of contacts, one set 195s of which functions, when the relay is deenergized, to lock the relay operating coil l194iacross the terminals 188 ,and 192 and the other set 195b of which energizes an alarm bell to indicate system breakdown.

Failure at the end transmitting stations can be due to one of two causes, either a loss of modulation or a loss of the carrier wave signal. In the event that only modulation signal is lost from the end station .12 and assuming ,that the rest of the system remains operative, the 600 cycle signal at the ouput of iilter 92 disappears and the tone detector relay 184 closes, `but the tone detector relay 186 remains open. Under these conditions the Green indicating light 187 comes on, but the circuit to the operating coil 194 is not broken, since this circuit is still completed through the closed contacts of the rel-ay 186. Therefore, the alarm bell does not sound. If, Ion the other hand, the end station 12 loses its carrier wave signal, this acti-on will soon -be followed by a loss in beat signal youtput `at the receivers 38 and 39 and, consequently, the end station 11 will lgo off the :air `at the `end of the delay oli period. Under these conditions signal output from both of the ilters 76 and 92 disappears and both of the relays 184i and 186 are closed to ybreak the circuit to relay coil 194 and, hence, to sound the alarm tbell.

Similarly, if the end station 11 should lose only modulation, signal ydisappears from the output of iilter '76 and the Red signal light 193 becomes illuminated but the alarm bell does not sound. Loss of carrier wave signal output at the end station 11 is, ofcourse, accompanied by a shutdown at the end transmitting station 12 to voperate the `alarm bell of the condition indicator 77. Thus, if one of the lights 187 or 193 is illuminated without sounding the alarm the operator is informed that the station identied by the particular light illuminated has lost its modulation. lf the alarm bell rings the operator at the center station can surmise that one of the end stations has lost its carrier and he may `determine the cause of the failure by interrogating the end stations. Interrogation of the :end stati-on 1l is performed as described 'above by rendering the center station effective by closing switch 96 to radiate its 1772.000 kilocycle signal and the 240 cycle signal output from the lamplifier control circuit is modulated upon the wave. If the end station 11 is functioning properly, it will come on the air `and its operation Will be indicated by the signal monitor circuit in the manner previously described. The interrogation :of the end station 12 may, of course, be eiieoted by closing switch 97 to cau-se the center station to radiate its 1798.000 kilocycle signal and by modulating this signal with the 600 cycle tone switching signal from -the oscillator control circuit 80. In this manner the source of the trouble, can be determined and the necessary steps can be taken to correct it. The alarm bell of the condition indicator remains energized even though the standby transmitter at the station which failed is rendered operative and the bell remains energized until the reset switch 191 is opened to 'break the circuit to the relay 195.

As indicated above, the automatic transfer between the transmitters 1d and l15 Iat the center station 10 is eiected by the transfer unit `16 in cooperation with the antenna changeover unit 23. The antenna changeover unit 23 is generally similar to the unit 33 illustrated in FIG. 2 and described above and includes tmeans for applying a sample RF signal to lan RF 'detector circuit 198 (FIG. 5) rin the unit 16 as, for example, through `a signal connector 199. This RF signal is applied through a radio frequency til-ter 200 Iwhich may Ibe tuned either to the 1772.000 kilocycle frequency or to the 1798.000 kilocycle frequency. The selected frequency iis passed through a half-wave rectilier circuit 201 having a long time constant RC lilter therein which recties the input signal and supplies negative half-cycles to the control grid of a DC. amplier 202. The long time constant RC iilter, which may be either at the output of the rectifier 201 or at the linput ofthe D.C. amplifier 202, Imaintains the negative signal for a period in excess of the switching period by closing switch 96 to radiate its 1772.000 kilocycle signal yand the 240 cycle signal output from the amplifier -control circuit 80 is modulated upon the Wave. 4If the end station 11 -is `functioning properly, it will come on the air and its operation Will be indicated by the signal monitor circuit 90 in the manner previously described. The interrogation yof the end station 12 may, of course, be effected by closing switch 97 to cause the center station to radiate its 1798.000 kilocycle signal `and by modulating this signal with the 600 lcycle tone switching signal from the oscillator control circuit 80. In this manner the source of the trouble can be determined land the necessary steps can be taken to correct it. The alarm bell of the condition indicator remains energized even though the standby transmitter at the station which failed is rendered operative and the bell remains energized until the reset switch 191 yis opened to break the circuit to the relay `195.

As indicated above, the 'automatic transfer .between `the transmitters 14 and 15 -at the center station 10 is eliected !by the transfer unit -16 cooperating with the antenna changeover unit 23. The [antenna changeover unit 23 is generally similar to the unit 33 illustrated in FIG. 2 `and described above and includes means -for applying `a sample RF signal to an RF detector circuit 198 (FIG. 5) in the unit 16 as, for example, through a l@ signal connector 199. This RF signal is applied through a radio frequency filter 200 which may be tuned either tot the 1772.000 kilocycle frequency or to the 1798.000 kilocycle frequ-ency. The selected frequency is passed through a half-wave rectifier circuit 201 having a long time constant RC filter therein which rectifies the input signal and supplies negative half-cycles to the control grid of a D.C. amplifier 2,02. The long time constant RC filter, which may be either at the output of the rectifier 201 or at the input of the DC. amplifier 202, maintains the negative signal for a period in excess of the switching period of the center transmitting station and, as a result, -a substantially continuous negative signal is supplied to the amplifier 202 as long as the center station is radiating V"its radio frequency signals. YThe negative signal supplied .to the amplifier 202, of course7 cuts off the plate circuit flow of `this amplifier and opensr the hiatus reducer relay 203. ln the event of failure of the output signal supplied from the antenna changeover unit 23, the negative voltage supplied to the control -grid of the DtC. amplifier 202 is removed and this amplifier starts to pass plate current through the operating coil 204 of the relay 203, thereby to close the relay 203 and apply the voltage from a suitable alternating current source such as a 110 volt supply to a time `delay relay 205. The latter relay is` indicated as being of the thermal type and at the end of the predetermined time delay period the relay 205 closes its .contacts and completes a circuit to energize lthe operating coil 206 of a ratchet transfer relay 207. The

ratchet relay 207 is similar to the relay 122 describedv above yin that energization of the operating coil 206 actuyates an operating pawl 208 to advance the ratchet relay one step. Movement of the ratchet of the relay 207 drives a shaft 209 to rotate switch operating cams 210 and 211 which function to alternately open and close switches' 212 and 213 in -an obvious manner. When switch 212 is closed, switch 213 is open, and supply voltage is delivered through the operating coil of an lantenna changeover relay 21-4 to the transmitter V14. The antenna changeover relay when energized connects the output of the final amplifier of the transmitter 14 to the antenna circuit 24 and, when deenergized, connects the output of transmitter 15 to the antenna circuit 24. When the ratchet relay 207 is so positioned that the switch 212 is opened, the switch 213 is closed and power is supplied through connector 215 to the transmitter unit v15. Under the latter conditions the antenna changeover relay 214 deenergized and the fina-l amplifier of the transmitter 15 is connected to the antenna circuit 24. A manually operated switch 216 may be employed to bypass the Iratchet relay 207 Iin order to place either the transmitter 14 or the transmitter 15 [in operation for purposes of testing or calibration. Thus, when the switch 216 is in the upper position lillustrated in FIG. 5, power supply to the transmitter 14 is `delivered through the antenna changeover relay and through the closed contacts of section216a of switch 216. No power, of course, flows to the'transmi-tter 15 in View of the open circuit at switch section 21617. When the switch 216 is moved to its lower position, section 216b of the switch 216 functions to supply plate voltage to the electron discharge tubes of the transmitter 15 and section 21621 interrupts the circuit to transmitter 14. The antenna changeover relay 214 is obviously controlled by the actuation of the sw-itch 216 to effect the connection of the proper transmitter to the antenna .circuit 24. In view of the foregoing description, it will be recognized that in the event that the signal input to the unit 16 fails, the relay 203 :is closed to operate the time delay relay and after the predetermined delay period has expired the ratchet relay 207 is advanced to automatically transfer the operation to the standby transmitter. The delay period effected hy therel-ay 205 is again provided in order tot prevent momentary failures from shutting downfthe equipment, thereby permitting 20' these momentary failures to correct themselves without interfering with the operation of the system.

'llurning now to the equipment comprising the failure indicator 70 which, as previously indicated, may be located either at the network office or at any point remote from Kthe transmitting stations/10, y11 land 12, this equipy ment comprises a pair of receivers 220 and 221, respectively tuned to frequencies `of 1772.120 and 1798.300 kilocycles. These receivers are excited [by `a common loop receiving antenna 222 which is so oriented that it accepts Ithe signals radiated from all three of the stations 10, 11 and 12. The Green receiver 220 is, of course, sufficiently selective to reject the signal radiated from the end station 11 and the 1798.000 kilocycle signal alternately radiated from the center station 10, while the Red receiver 221 is sufficiently selective to reject the sigrnal continuously radiated 'from theend station 12 and the 1772.000 kilocycle signal alternately radiated from Ithe center station 10. The receiver 220 heterodynes the signal `from the end station 12 with the 1772.000 kilocycle signal alternately radiated from the center station 10 to :develop a 240 cycle beat lfrequency which is passed through a band pass filter 223 to a Green heterodyne tone detector 224. The Green receiver 220 also reproduces the 600 cycle reference signal alternately modulated upon the carrier wave continuously radiated from the end station 12 and passes this` 600 cycle signal through a Ihand pass filter 225 to a Red modulation tone detector 226. The filter 225 is, of course, 'sufficiently selective toreject the 240 cycle beat frequency signal developed by the receiver 220 and, correspondingly, the filter 223 rejects the 600 cycle reference signal reproduced by the receiver 220. The Red receiver 221 heterodynes the signal continuously radiated from the end station 11 with the 1798.000 kilocyc'le signal alternately radiated from the center station 10 to develop a 600 cycle beat frequency signal which is passed through filter 227 to a Red heter-odyne tone detector circuit 228. rlihe Red receiver 221 Ialso reproduces the 240 cycle reference signal alternately modulated upon the carrier wave continuously radiated from the end station 11 and applies this signal through a 240 cycle band pass filter 229 to a Green modulation tone `detector 230. The filter 229, of course, rejects the 600 .cycle heterodyne signal developed by the receiver 221 and the filter 227 rejects the 240 cycle reference signal reproduced by the receiver 221.

The tone `detectors 224, 226, 228 land 230 are similar to the tone detectors 74 .and 75 illustrated in FIG. 4 and each functions to hold open a tone detector relay as long as it is excited by input signals of the proper frequency. Of course, each of these tone :detectors contains an audio frequency circuit tuned to the appropriate frequency to prevent its operation by intermittent or spurious signals. The tone detector relay associated with the tone detector 224 is indicated at 22451, the tone detector relay lassociated with the tone `detector 226 is indicated at 22621, and so forth.

There are four conditionsv which 'can exist at the end stations to cause operation of the system to break down, and these 'four conditions may be analyzed .and identified by reference -to the four signals supplied -to the four tone detector circuits of the failure indicator` '70. Failure indications respectively lrepresentative of the four conditions are provided lby employing the tone detector relay to energize a relay matrix indicated generally at 231 in the manner described below. When the system is operating properly all four fof the described tones are Suppliedto the tone detectors and yall lof the relays are open. End station failure can occur in any one of the following four ways: Y

Condition 1.-'Iihe Green station carrier fails, in which event the Green heterodyue signal disappears from the output off the filter 223, the Green reference signal is no longer developed by the receiversk 3S and 39 at the end station 11 and, accordingly, the Green reference signal does not appear at the output of the lfilter 229. The Red reference signal no longer appears at the output of the lter 225, since the carrier wave for this signal is no longer available. At the end station lll, development of the 240 cycle beat signal by the receivers 38 and 39 terminates, but the Red stati-on carrier remains on the air for 90 seconds before the delay off relay is operated. During this 90 second period, the Red heter-odyne signal is developed at the output of the band pass filter 227 and the tone detector 228 maintains its relay open while, as indicated above, the relays `associated with the other three tone detectors 224, 226 and 23) are all closed.

Condition 2 The Green station modulation fails, in which case the Red reference signal appearing at the output of the band pass lter 225 disappears. Since all carriers are still present, both the Green and Red heterodyne signals and the Green reference signal are available and are passed to their associate-d tone detector circuits. Under these conditi-ons the relay associated with the tone detector 226 is closed, while the relays associated with the other three detectors remain in their open position.

Condition 3.--The Red station `carrier fails, in which event the Green heterodyne signal is present for 90 seconds following the failure, while the Red lieterodyne signal, the reference signal andthe Green reference signal all disappear. Thus, under these conditions the relay associated with the tone detector 224 is open, while the relays associated with the other three tone detectors are closed.

Condition 4.-The Red station modulation fails, in which case the Green reference signal disappears from the output of the band pass filter 229. The Green heterodyne signal, the Green reference signal and the Red reference signal are all present. Under these conditions the relay '23% lassociated with the tone detector 230v closes, while the relays associated with the other three tone detectors remain open.

As previously indicated, it is the function of the relay matrix '231 to respond to these on-o conditions of the relays by indicating which of the four enumerated conditions is present in the event of end station failure. The indications are provided in the form of signal lights 232, 233, 234 and 235, which respectively indicate the presence of conditions 1, 2, 3 and 4 enumerated above. rThe relay matrix 231 is excited Ifrom -a 110 volt supply appearing between line conductors 236 and 237. The presence of excitation voltage 'between these two conductors is indicated by a power-on light 238. The excitation voltage for the D.C. power supply and for the other circuits of the failure indicator 70 is derived through an ori-olf switch 239 `and through ka signal connector 240. Line conductor 237 is connected through ia manually operated reset switch 241 to one side of each of the signal lights 232, 233, 234v and 235, and to one Iside of 4a systemon 'light 242 which has its other side connected through the normally closed contacts of a suitable time delay relay 243 to the other line conductor 236. Thus, the system-on light 242 provides an indication that the contacts of relayZl-S are closed and that the relay matrix 231 is conditioned for operation.

The line conductor 237 is also connected to the movable arm or pole of each of the tone detector relays 224e, 22611, 22851 and 23h11. The normally open contacts of the latter relays are respectively connected to one side of the operating coils of relays 25o, 251, 252 and 253y in the matrix 231. The other sides of these relays are connected together to the movable arm of the time delay relay 243 and through the normally closed contact 243b of this relay to the line conductor 236. Thus, as soon as one of the tone detector relays is closed by the absence of signal at the input circuit of its associated tone detector, a circuit is completed for energizing the associated matrix relay. Specifically, under condition 1 described above,` relays 256, 251 and 252 are simultaneously energized to close their contacts. Closing of contacts 25th: and 251e con- 75 nects the line conductor 236 through connector 254 to the signal light 232 in order to indicate that the Green station carrier wave has failed. Connector 254 also functions to connect the line conductor 236 through conductor 255 to parallel connected operating coils of relays 256 and 257 in the matrix, the other end of both of these relays being connected directly to line conductor 237. Operation of relay 257 opens its contacts and prevents the light 235 from being turned on. Light 234 cannot be turned on because it is confronted by an `open circuit `at contact 25311 of relay 253, while light 233' is not illuminated because it is confronted by an open circuit at contact 251b of relay 251.

Energization of the relay 256 breaks the normally closed Contact 256b and, hence, interrupts an energizing circuit for the operating coil of an alarm relay 261. This energizing circuit extends from line conductor 236 through normally closed contacts 256b, through normally closed contact 258b, through normally closed contact 25% and through normally closed contact 260b. Deenergization of the `operating coil of the relay 261 causes the movable arms of this relay to occupy the position illustrated in FIG. 6 so that contact 26141 is closed to complete a circuit to an alarm not shown. The alarm, of course, apprises the operator at the failure indicator station that normal operating conditions no longer prevail. Closing of contact 26lb of relay 261 connects the line conductor 237 across the operating element 24311 of the time delay relay 243. If the cause of the operational failure of the Green carrier does not remedy itself within the delay period of the relay 243, :this relay becomes energized to break Contact 243b and, hence, to remove operating potential from the relays 250, 251, 252- and 253. lf, on the other hand, the cause of the failure is remedied before the cycle of relay 2143 has been completed, the tone detector relays all revert to their open conditions and the relay 261 is again energized to break the circuit to time delay relay 243 and to shut olf the alarm. The time delay provided by relay 243 lis less than that provided by the transfer delay relay employed at the end stations.

Operation of the relay 256 has another important result, in that it closes contact 25611 and completes a circuit to the operating coil of relay 256 from conductor 254 through connector 255 in order to lock this operating coil across the voltage supply line. Cessation of current flow in the relays 250, 251, 252 and 253 insures that further events taking place at the tone detectors have n0 effect, due to the fact that the contacts of the time delay relay 243 are open when the relay 261 isdeenergized. Thus, even though the standby transmitter at the end station which failed is subsequently rendered effective, this can have no effect on the failure indicator 70. Specifically, the contacts of the time delay relay 243 are maintained in their open position, due to the fact that the operating element 24361 is locked directly across the line conductors 236 and 237. The failure indicator also functions to maintain the signal light 232 illuminated in view of the fact that the coil of relay 256 is locked across the line and the now closed contact 25641 is effective to complete the circuit from line conductor 236 lthrough conductor 254 to the light 232. Thus, even though the operator may be temporarily absent when the failure occurs, the failure indicator maintains its indications to inform him that repairs must be made at the identified end station.

To reset the unit for operation after the failure has been corrected, either by Way of corrective repairs or otherwise, it is necess-ary merely to open the manually operable reset switch 241 which breaks the circuit to all of the signal lights and, at the `same time, interrupts the circuit to the operating element 24311 of the time delay relay 243. Operation of the reset switch also breaks the energizing circuit for the relays 256 and 257 and allows the contacts of these relays to return to the position illustrated in FIG. 6. Thus, when the reset switch 241 is again deenergized state.

23 closed,rall of the circuit elements of the failure indicator 70 again occupy the positions illustrated in FIG. 6.

If the second condition prevails, relay 252 is energized to complete a circuit'from line conductor 236 through its closed contact 252b, through the normally closed contact 251b, through signal connector 262 and through the normally closed contact 263a of relay 263 to .the Green modulation signal light 233. Signal light 232 remains off, since it is confronted by an open circuit at contact 25ml, signal light 235 remains off because its energizing circuit is open at contact 250b and signal light 234i remains off because it is confronted by yan open circuit at Contact 253er. j The energized conductor 262 is connected to one end 'of the operating coil of relay 25S so that the latter relay is actuated to open its contacts in order to lock the operating coil across the line by closing of contact 25861 and to interrupt the energizing circuit for the operating coil of relay 261 by breaking contact 258b. Deenergization of the operating coil of relay 261, of course, actuates the alarm circuit and, at the `same time, applies an operating voltage across the operating element 243a of the time delay relay, 243 with the results described above.

Under condition 3 described above, relays 250, 252 and 253 are all actuated, while relay 251 remains in its Energization of the three identified relays completes a circuit from the line conductor 236 ,through the now closed contact 253g, through the now closed contact 252a and through signal connector 265 to illuminate the Red carrier signal light 234. The energized conductor 265 applies operating voltage across the operating coils of relays 259 and 263. Actuation of the relay 263 `opens its contact 263er and prevents the signal light 233 from being illuminated. The energizing circuit for signal light -235 is open at contact 253]: and, hence, light 235 remains off. 'Ihe circuit to signal light 232 is open at contact 25i1a and, as a result, this light does not become illuminated. Operation of the relay 259 -breaks the contact 259b to interrupt the circuit to the alarm relay 261 and, at the same ltime, closes contact 259:1 to lock the operating coil 259 across the line, with the resultsdescribed above.

Under the fourth condition, the relay 250 is energized, While the relays 251, 252 and 253 remain in their deenergized conditions. When relay 250 is actuated, a circuit is completed from line conductor 236 through normally closed contact 25311, through the now closed contact 25017, through conductor 270, through the closed contacts of relay 257 and through conductor 271 to the signal light 235. The light 235 is thus illuminated to in- Adicate that the Red station modulation has failed. The signal light 232 is not illuminated, in view of the fact that its energizing circuit is opened at Contact 251a while the Vcircuit for light 234 is open at contact 252:1 and the lenergizing circuit for light 233 is open at contact 25217.

Application of energizing potential to the conductor 270 causes an operating current to be passed through the coil of relay 260 in order to actuate the contacts of the latter relay. yOpening of contact 2601), of course, breaks the energizing circuit for the relay 261 with the results described above while closing of contact 26051 locks the coil of relay 260 across the line, in view of the fact that the contact 26061 is connected directly to the line conductor 236.

In view of the foregoing description, it Will be recognized that the failure indicator 70 functions to provide an immediate indication of operating failure occurring at either of the end stations 11 or 12. The lock-on feature of the relay matrix 231 maintains the indication until the trouble has been corrected and until the reset switch 241 is opened manually.

The system shown in FIGS. l to 6 inherently provides means for Calibrating or checking the system for phase shifts either in the transmitting equipment or in the receivingiequip-ment. Specilically, if the switches 79 and 97 are operated to apply a 600 cycle signal to the modulator 19 for modulation upon the 1798.000 kiiocycle wave from the oscillator 18, this modulated signal Will be received at the Green-end station 12 and after the delay on period described above the 600 cycle modulation signal reproduced from the incoming signal will be modulated upon the 1772.240 kilocycle signal radiated from the Green station. The tmobile receiver detects both of the 600 cycle modulation components and applies them to the Green phase indicator which measures the relationship therebetween. The phase indication provided will be the same regardless of the position ofthe mobile receiver except, of course, for small differences in travel time of the waves traveling from the stations 10 and 12 to the mobile receiver. Any such differences can be calculated for the receiver location. Thus, as the mobile receiver progresses during a survey, the system may be periodically shut down and the 600 cycle reference signals may be radiated from the stations 10 and 12. Any major changes in the `Green phase indication provided at the mobile receiver will inform the operator that a phase shift has occurred either in the equipment at the mobile receiver or in the equipment at one of the transmitting stations 10 `or 12. The same procedure may be used .to check for phase shifts in the Red indicating system by radiating the 240 cycle` reference signals from the stations 10 and 11.`

ln view of the detailed description above, it will be recognized that the system illustrated in FIGS. l through 6 may be turned on or off from :the center transmitting station 10 and, accordingly, the present invention avoids the use of a continuous station Watch at the end transmitting stations 11 and 12. Moreover, all three of the transmitting stations include -standby transmitting equipment which is automatically rendered operative in the event of failure of the main transmitting equipment. Furthermore, the system lincludes means at the center station and at the failure indicator 7i) for monitoring the system in such fashion that operational failure is immediately indicated and also for identifying and locating the source of the trouble.

The invention may also be employed in a transmitting system of the type disclosed and claimed in U.S. Patent No. 2,513,316 to I ames E. Hawkins, assigned to the same assignee as the present invention. Such a system is illustrated in FfGS. 7 and 8, wherein there is 4shown a center transmitting station 310 and a pair of spaced apart end transmitting stations 311 and 312, which are adapted to radiate signals of dierent frequency separated from each other by a small audio frequency difference. The end stations 311 and 312 are preferably iocated approximately equal distances from the center station 310 and are so disposed that an imaginary base line extending between the center station 310 and the end station 311 is angularly related to a similar base line extending between the center station 310 andthe end station 312. Tne transmitting system also includes a reference station 313 located at some distance from the three transmitting stations 310, 311 and 312 and effective to transmit reference signals in the form of modulation signals developed from heterodyning waves received from the stations 310, 311 and 312. y

Equipment is employed for turning on and off three of the stations 310,- 311 and 312, under the control of modulated signals radiated from the reference station 313. In addition, each of the four transmitting stations includes standby transmitting equipment which is automatically rendered effective in the event of failure of the main transmitting facilities.

Furthermore, monitoring equipment is provided at the reference station 313 for indicating failure of one of the transmitting stations and for ridentifying the station at which the failure occurred. This indication is maintained even after the standby transmitting equipment has been rendered operative, so that the necessary repairs can be made Without requiring the system to be shut down.

The equipment provided at the center station 310 to effect the above enumerated results comprises a first transmitter 314 and a second transmitter 315, only one of which is adapted to be rendered operative at a time, so that the inoperative transmitter serves as a standby unit. Each of these transmitters includes an oscillator or signal generator, a final amplifier and one or more intermediate amplifiers and each is adapted to develop a signal having a frequency of 1798.240 kilocycles. The output signal from the final amplifier of both of the transmitters 314 and 315 is supplied through an antenna changeover unit 323 to an emitting antenna circuit 324. The antenna changeover unit 323 is identical to similar units described above and functions automatically to connect the antenna circuit 324 to the output of the proper transmitter 314 or 315 in the manner previously described. The antenna changeover unit 323 also supplies a sample of the RF output signal supplied to the antenna circuit 324 through a signal connector 325 to a transfer unit and RF detector 316. The latter unit is similar to the transfer unit illustrated in FIG. 2 and employed at the end station of the transmitting system illustrated in FIG. 1. However, the transfer unit 316 includes an RF detector like the detector 198 illustrated in FIG. 5 in place of the modulation detector 138. This RF detector, of course, functions to control the operation of a relay to maintain the relay open as long as an RF signal is being supplied to the antenna 324 and to close the relay in the event of signal failure. This relay corresponds to the relay 137 of the transfer unit shown in FIG. 2. In all other respectsv the transfer unit 316 is identical to the unit 32 described in detail above.

'llhe remote control at center station 310 is effected through control of the transfer unit 316 by means of a tone detector 317 which is identical to the tone detector 74 illustrated in FIG. 4. 'Ilhe tone detector 317 is excited by the signals developed by a continuously operated re- 4ceiver 318 tuned to a frequency of 1772.000 kilocycles and sufficiently selective to reject signals from the end stations 311 and 312. The signal input terminals of the receiver 318 are excited by the signals picked up by a loop receiving antenna 320 which is so oriented that the signals emitted from the emitting antenna 324 are nulled, While the signals radiated from the reference station 313 are accepted. The signals developed by the receiver 318 are passed through filters 321, 322 and 326 respectively tuned to frequencies of 240 cycles, 360 cycles and 600 cycles to a non-linear mixer circuit 327. The output signals of the latter mixer circuit are passed through a band pass filter 328 tuned to a frequency of 600 cycles to the tone detector 317. Standby or alternate receiving equipment may, of course, be employed if desired, but this is not illustrated in the drawings.

The equipment provided at the end station 311 comprises a pair of transmitters 330 and 331, each developing a signal having a frequency of 1798.000 kilocycles. Only one of these transmitters is employed at a time, the other unit serving as a standby transmitter in case of operational failure of the main transmitting equipment. Each transmitter includes an oscillator or signal generator and one or more amplifying stages, the final amplifying stage of each transmitting unit being connected to deliver its output signals through an antenna changeover unit 333 to an antenna circuit 334. 'Ilhe antenna changeover unit 333 is like the unit 33 employed at the end station 11 in the system illustrated in FIG. 1 and includes lmeans for automatically applying the output ofthe proper transmitter 330 or 331 to the antenna circuit 334. Transmitters` 330' and 331 are rendered effective automatically by means of a transfer unit and RF detector 332 which is excited by a sample signal supplied from the antenna changeover unit 333 via a signal connector 335. The unit 332 is identical to the unit 316 employed at the end station and is controlled by a tone detector 336 which corresponds to the detector 317 at the center station. The tone detector 336 is excited by the signals developed by a receiver 338 and passed through a 240 cycle band pass filter 339. The input terminals of the receiver 338 are excited by the signals picked up by a loop receiving antenna 340 which is so oriented that a maximum response is provided for signals arriving from'the reference station 313, while the high power signals emitted from the antenna 334 are effectively nulled or eliminated. The receiver 338 is center tuned to a frequency of 1772.000 -kilocycles and, thence, is adapted to reject the signals radiated from both of the stations 310 and 312, while accepting the signals radiated from the reference station 313.

The equipment provided at the end station 312 comprises a pair of transmitters 350 and 351 for developing signals having a frequency of 1798.600 kilocycles, which are passed through an antenna changeover unit 353 to a radiating antenna circuit 354. Only one of the transmitters 350 or 351 is effective at any particular time, the remaining unit being a standby transmitter Which is rendered operative automatically in the event of failure of the effective transmitter under the control of a transfer unit and RF detector 352. 'Ilhe transfer unit 352 is identical to the unit 316 employed at the icenter station and is excited by RF signals passed from the antenna changeover unit 353 through a signal connector 355 to the RF detector portion of the unit 352. Remote control of the end station 312 is effected by controlling the operation of the transfer unit 352 through a tone detector 356 similar to the tone detector 317 referred to above. The tone detector 356 receives excitation from the output signals of a receiver 358, which signals are passed through a band pass filter 359 tuned to a frequency of 360 cycles. The signal input terminals of the receiver 358 are excited by the signals picked up by a loop receiving antenna 360 which is so oriented that the signals emitted from the antenna 354 are nulled or eliminated. rllhe receiver 358 is center tuned to a frequency of 1772.000 kilocycles and its input stages are sufficiently selective to reject the signals arriving at the antenna 360 from the center station 310 and from the end station 311, While passing the signals -arriving from the reference station 313.

Turning now to the equipment provided at the reference station, this equipment includes a pair of transmitters or signal generators 370 and 371, each of which includes an oscillator developing signals having a frequency of 1772.000 kilocycles, an amplitude modulation circuit and a final amplifier stage. 'Ilhese transmitters are, of course, rendered operative one at a time and the inoperative transmitter functions as a standby unit. The final amplifier stage of the operative transmitter lis automatically connected to an antenna circuit 373 by means of an antenna changeover unit 372 like the unit 323 described above. Automatic transfer to the standby `transmitter in the event of failure of the operative transmitter is effected by a transfer unit and modulation detector. 374. The unit 374 is similar to the transfer unit illustrated in FIG. 5, but it includes a modulation detector circuit like the circuit 138 shown in FIG. 2 in place of the RF detector circuit 198. Ilhe modulation detector circuit is excited by signals supplied from the antenna changeover unit 372 and, in the absence of such signals, the detector relay at the output of the modulation detector circuit functions to effect the automatic transfer to the standby transmitter in the manner previously described. l

To provide reference signals for phase comparison at the mobile receiving unit with heterodyne signals derived from Waves radiated from stations 310, 311 and 312 in conventional manner, the reference station 313 includes a reference receiver 375 which is center tuned to a frequency of 1798.300 kilocycles and which accepts the signals radiated from all of the stations 310, 311 and 312. To this end, the signal input terminals of the re- 

5. IN A TRANSMISSION SYSTEM OF THE HYPERBOLIC TYPE, AT LEAST THREE SPACED APART TRANSMITTING STATIONS RADIATING SIGNALS OF DIFFERENT FREQUENCY, MEANS FOR RECEIVING ALL OF SAID SIGNALS AT A COMMON POINT, MEANS RESPONSIVE TO THE RECEIVED SIGNALS FOR CONTROLLING THE OPERATION OF A PLURALITY OF RELAYS, A PLURALITY OF INDICATING DEVICES AND A SYSTEM FAILURE INDICATOR, SAID RELAYS BEING CONNECTED IN A RELAY MATRIX CIRCUIT FOR CONTROLLING THE ACTUATION OF SAID DEVICES AND SAID INDICATOR IN SUCH MANNER THAT OPERATIONAL FAILURE OF ANY OF SAID STATIONS RENDERS SAID INDICATOR EFFECTIVE AND ALSO RENDERS EFFECTIVE ONE OF SAID DEVICES TO IDENTIFY THE STATION AT WHICH THE FAILURE OCCURRED, TIME DELAY MEANS FOR PREVENTING SAID INDICATOR AND SAID ONE INDICATING DEVICE FROM BEING RENDERED EFFECTIVE UNLESS SAID FAILURE EXISTS FOR A PREDETERMINED PERIOD, MEANS FOR MAINTAINING SAID INDICATOR AND SAID ONE DEVICE EFFECTIVE FOLLOWING SAID PREDETERMINED PERIOD EVEN THOUGH THE STATION WHICH FAILED RESUMES OPERATION, AND MANUALLY OPERATED RESET MEANS IN SAID CIRCUIT FOR RENDERING THE INDICATOR AND SAID ONE DEVICE INEFFECTIVE AND FOR RESETTING THE CIRCUIT TO PERMIT INDICATIONS CONCERNING SUBSEQUENT FAILURES. 